Lib MPs lobbied hard about gay marriage

Both sides of the same-sex marriage debate are furiously lobbying Liberal MPs ahead of a Senate debate on the issue.

But they may be wasting their time because a free vote isn't likely to be discussed at a Liberal party room meeting on Tuesday, the last chance before budget week in May.

The Australian Christian Lobby arranged for two million emails from 20,000 Australians to be sent to Liberal MPs urging them to vote as one to preserve existing marriage laws.

The Senate is scheduled to debate a private bill to legalise gay marriage, proposed by Liberal Democrat David Leyonhjelm, on Thursday.

Liberal senator Chris Back dismissed as a "furphy" the need for the party room to allow a conscience vote, arguing backbench MPs and senators already enjoyed the freedom to cross the floor and vote against the party.